Amplifying system



Dec. 10, 1940.

J. O. EDSON AMPLIFYING SYSTEM Filed Aug. 2 1958 ATTORNEY Patented Dec.10. 1940' Nrreo STATES ALIPLIFYING SYS TEM .Iames 0. Edson, Great Kills,N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York,N. 2., a corporation of New York ApplicationAugust 2, 1938, Serial No.222,648

3 Claims;

This invention relates to an electric wave amplifying system, and moreparticularly, to an automatic volume control arrangement therefor.

An object of this invention is to satisfy the 5 need for an amplifyingsystem that will amplify signal waves over a wide range of frequenciesand that will have a substantially constant output regardless ofrelatively large variations in its input.

A feature of the invention comprises an amplifying system in which thesignal output operates a control device associated with an amplifyingdevice of the system to vary in predetermined manner the gain of theamplifying device in accordance with variations in such signal outputwhereby the amplifying systems output is substantially constant for widevariations in its input.

Another feature comprises such an amplifying system in which the controldevice comprises an 20 electron discharge device operated as a detectorwith a very high grid bias and circuit arrangement such that the workingpoint of the detector is at the point of maximum slope of the platecurrent versus alternating current input characteris- 25 tic of thedetector.

A further feature comprises suchan amplifying system in which anytendency for the control grid of the amplifying device to go positivebecause the anode of the control tube is at a higher potential than theamplifying device cathode, is neutralized.

Other and further features will be evident from the general and detaileddescription which follows.

In accordance with the invention, the amplifying system comprises athree-stage amplifier having a pentode-diode for the first stage andpentodes for the other two stages, the stages being coupled throughinterstage networks or circuits having a substantially fiat frequencycharacteristic over the range of frequencies to be amplified, forexample, up to 150 kilocycles. When operating at normal load, the outputof the amplifier is maintained substantially constant for relativelylarge variations in' the amplifier input, by means of a control tube andassociated circuit.

The control tube is a pentode, preferably, with obtained through apotential divider from the.

plate supply common to all of the tubes. The anode of the control tubeis connected to the grid circuit of the controlled tube. When the signalinput, 1. e., the amplifier output, is sufficiently great to.overcomethe control tube grid 5 bias sufiiciently to permit plate current of anorder reducing the plate potential to a point where it is slightly lesspositive than, or more negative with respect to, the cathode of thefirst tube of the amplifier, tendency toward increases 10 in theamplifier output results in increase in the negative bias on the controlgrid of the amplifiers first tube, and a loweringof the gain of theamplifier so that the amplifier output returns to the original levelfrom which it departed because of 15 an increase in the input to theamplifier. A tendency to a decrease in the amplifier output from itsdesired level causes thecontrol tube anode potential to increase, sincethe plate current decreasesrand the control grid bias of 'the'controlledtube to become less negative whereby the gain of the controlled tube andof the amplifier increases to maintain the amplifier output at thedesired level.

When the amplifier output is such that'no plate current flows in thecontrol tube, or the plate current is such that the anode potential ofthe control tube is more positive than the cathode of the controlledtube, the control grid of the controlled tube may be driven positive. Toprevent this, the diode incorporated in the first tube of the amplifieris utilized, the anode of the control tube being connected to the anodesof the diode to produce a. potential drop between the control tube anodeand the point at which it is connected to the grid circuit of thecontrolled tube.

A more completeunderstanding ofthis invention will be obtained from thedetailed description which follows, taken in conjunction with theappended drawing, the single figure of which shows an electric waveamplifying system embodying the invention.

The single figure shows a three-stage amplifier comprising an inputtransformer 10, an output transformer II, electron discharge devices l2,i3, 14, interstage coupling circuits or networks l5, l6, and a controlcircuit I'I including an-electron discharge device la. The device I2 isa pentodediode tube, for example, an R. C. A. type 637 tube, devices l3and I4 are pentodes, for example, Western Electric type 310A and 311A,respective1y,-and device I8 is a pentode, for example, Western Electrictype 310A, each tube having an indirectly heated cathode.

The primary winding of the input transformer is connected to the inputterminals 28. Its secondary winding is connected to the control grid 2|of tube l2, and through grid biasing battery 4 22 and high frequencyby-passing condenser 23 to the cathode 24. A resistance 25, of suitablesize for the transformer, is connected across the secondary winding sothat the amplifier input impedance approximates a pure resistance. Thecathode 24 is also the cathode for the diode forming a part of tube I2,the condenser 23 being connected between the cathode and the diodeplates 26. The suppressor grid 21 is connected directly to the cathode.The screen grid 28 is connected through condenser 28 to the ground endof the parallel connected resistance 38 and condenser 3| in the cathodelead. and to battery 33, which constitutes a common plate and screengrid supply for the tubes of the amplifier. A resistance 34 is connectedbetween the cathode l2 and the battery 33. The anode or plate 35 isconnected through resistance 36 and coil 31 to battery 33.

The tubes .l2, l3 are coupled through an impedance interstage orcoupling circuit comprising the output circuit of device l3, couplingcondenser 38 and input resistor 39 of the second stage of the amplifier,the elements 35, 31, 38, 39 being proportioned to have a substantiallyfiat characteristic over the frequency range being amplified.

The electron discharge device |3 comprises a cathode 40, control grid4|, screen grid 42, suppressor grid 43 connected directly to thecathode, and anode 44. Grid biasing potential is provided by cathoderesistor 45. Battery 33 supplies positive potential for the screen grid,and through output coil 45 for the anode.

The tubes l3 and 4 are coupled through an 40 impedance interstage orcoupling circuit |6 comprising the output circuit of tube l3, couplingcondenser 41 and input resistor 48 and coil 49 of tube l4, the elements46, 41, 48 and being proportioned to have a substantially flat fre- 45quency characteristic over the frequency range being amplified.

The tube l4 comprises a cathode 50, control grid 5|, screen grid 52,suppresser grid 53 connected directly to the cathode, and anode 54.

5 Control grid biasing potential is provided by battery 55. Condenser.55 is connected between the cathode and the screen grid, the latterbeing connected to the positive polev of battery 33.

The anode 54, also, is connected to battery 33 55 through the primarywinding of the output transformer II, the secondary winding of which isconnected to output terminals 68. Connected between the anode of tube I4and the cathode end of resistor 45, is a resistance 58 and a con- 60denser 59 in series.

The primary winding of a transformer BI is connected across thesecondary winding of the output transformer. The secondary winding oftransformer BI is connected to the control grid 65 52 of tube l8 andthrough potentiometer 63, slider 64 and serially connected biasingbatteries 65 and 55 to the cathode 66. A resistance 51 and coil 88 areconnected between the control grid and the potentiometer, the resistanceof 83 being 70 small relative to that of 61 and its variation permittinga slight control on the input voltage to the control tube. Thesuppressor grid 68 is connected directly to the cathode andthe screengrid 14 is connected to the positivepole of battery 33. Anode 18 isconnected through the input terminals connected parallel arrangedresistance 1| and condenser 12 to battery 33 and through filteringresistance 13 to the diode plates terminal of the condenser 23.

The amplifier described is adapted to have its to a source of highfrequency signal wave, for example, to the output of an oscillator, toamplify the received signal with a substantially fiat frequencycharacteristic over a desired frequency range, for example, up to 150kilocycles, and tosupply at its output terminals the amplified signal,the output voltage across the output terminals being constant for widevariations of the input voltage, 1. e., the output will vary less than.01 decibel for a variation of l decibel in the input.

This result is attained by the use of a control circuit including a gridbias detector having a very high grid bias together with a circuitarrangement such that the working point is at the point of maximum slopeof the plate current versus alternating current input characteristic ofthe detector.

The negative bias on the control grid, provided by batteries 55 and 65,is very much higher than that required to bias the tube |8 to cut-off.The peak input to the tube must, therefore, be considerably greater thanextinction voltage of the tube before any plate current flows in thecathode-anode circuit. When the amplifier is in normal operation, andvoltage at the output terminals of the amplifier rises above the desiredlevel, the peak input potential will overcome the bias on the controlgrid 82 and the detector will draw current. This will result, because ofcurrent flow through resistance 1|, in the potential between the anode10 and ground becoming less positive or more negative than when therewas no effective signal input to the tube I8. This variation may be andis used to vary, or, specifically, to increase the bias on a remote orcontrolled tube, specifically tube l2, and thus to reduce its gain, andtherefore, that of the amplifier, to lower the output signal to withinthe preassigned limits.

To obtain this control, however, the cathode of tube l8 should benegative with respect to that of tube l2. This result is obtained inthis amplifier by the connection of cathode 66 to ground, and theraising of the potential of the cathode 24 of tube l2 abovegroundby'means of a potential divider across the plate voltage supply33. Resistances and 34 constitute this potential divider. With thecathode 24 an appreciable potential above ground with respect to cathode66, it is apparent that if suflicient signal input is applied to-thecontrol grid 82, the anode 10 of tube lB will become negative withrespect to the c'atl'iode'of tube l2.

So far as operation with a steady applied signal is concerned, thearrangement just described would be sufficient to provide satisfactoryautomatic volume control. It is not desirable, however, to permit thecontrol grid of the controlled tube to be driven positive when thesignal output of the amplifier is insufficient to cause the control tubeto operate. The diode-circuit is provided for this contingency as is thebiasing battery 22 which provides a small initial negative potential tothe grid. If the signal output of the amplifier is insufiicient to causeplate current to fiow inthe control tube, the anode voltage of tube l8rises to a value morepositive than that of the cathode of tube l2 andcurrent begins to flow to the diode anodes 26 ,of the tube l2. Since theresistances 13 and 1| are large compared to the resistance of the diodefor positive voltage applied to the diode anodes 26, the drop across thediode is small. Thus the potential applied to the grid 2| of tube I2does not become positive with respect to its cathode at any time. Theresistances H and 13 are much larger than resistance 30 so that thecurrent to the diode does not produce any noticeable change in potentialacross resistance 30. If the signal input increases to the point whereplate current fiows in the control tube, the potential difierencebetween the diode plates .and the cathode decreases. As the platecurrent in the control tube increases, the potential of anode i0eventually becomes exactly the same as that of cathode 24. Signal inputcausing greater current flow produces no further eifect on the diode,since the diode plate potential falls below that of cathode 24, andcontrol of the grid bias of tube I2 begins. Thus, under actual workingconditions, when the control circuit is controlling the output of theamplifier, the diode has no eifect on the amplifier performance, but iseffective, when the control tube is not in control, to prevent thecontrol grid of tube i2 from swinging P sitive. The arrangementdescribed permits tube I8 to be operated in a range such that the slopeof the plate current versus alternating current input characteristic isquite large, increasing the direct current amplification through thecontrol circuit and increasing the degree of control obtained.

The serially connected resistance 58 and condenser 59 are proportionedto feed back a part of the output of the tube IE to the cathode resistor45, in such sense that the gain around the last two stages of theamplifier is reduced to stabilize the gain,'and to reduce distortion andnoise, in accordance with the teachings of H. S. Black Patent 2,102,671,issued December 21, 1937.

When a very stable source of plate potential is available, the use ofthe voltage divider to supply the potential difference between thecathodes of tubes l2 and I8 is satisfactory. If the plate supply is notvery stable, it may be necessary to use a dry cell battery to maintainthe cathode of tube 18 negative with respect to that of tube l2. Thiscan always be done with reasonable satisfaction because the currentdrawn by the control tube is never very large.

In an amplifier constructed in accordance with this invention, the platesupp y was 150 volts, the cathode 24 was maintained at about 50 voltsabove ground, resistances an and :4 being about 6,000

What is claimed is: v

1. In combination, an amplifier, an input circuit and an output circuittherefor, said amplifier comprising an amplifying electron dischargedevice having a control grid, a cathode and an anode, and a volumecontrol circuit connected between said output and input circuits andincluding a control electron discharge device having a control grid, acathode and an anode, said control devices grid being biased to anegative potential much greater than required for anode current cut-01f,its cathode being at a lower potential relative to ground than thecathode of said amplifying device, and its anode being connected to thegrid circuit of said amplifying device, and means to prevent theamplifying devices grid from going positive when the control devicesanode is at a higher potential than said amplifying device's cathode,said means being included in said amplifying device.

2. In combination, an amplifier, an input circuit and an output circuittherefor, said amplifier comprising an amplifying electron dischargedevice having a control grid, a cathode and an anode, and a volumecontrol circuit connected between said output and input circuits andincluding a control electron discharge device having a control grid, acathode and an anode, said control device's grid being biased to anegative potential much greater than required for anode current cut-off,its cathode being at a lower potential relative to ground than thecathode of said amplifying device, and its anode being connected to thegrid circuit of said amplifying device, and means to prevent theamplifying devices grid from going positive when the control device'sanode is at a higher potentialthan said amplifying device's cathode,said means comprising a diode, the anode of which is connected to thatof said control device.

3. In combination, a three-stage amplifier and a volume control circuittherefor, the first stage of said-amplifier comprising anamplifying-rectifying electron discharge device and the succeedingstages amplifying electron discharge devices, each device comprising acontrol grid. a cathode and an anode, and said control circuit includinga control tube having a control grid, a cathode and an anode, means forapplying an initial bias on the control grid'of the first device, acathode resistor in the second device for applying a bias to its controlgrid, and means for applying an initial bias to the control grid of thethird device and to that of the control tube, a negative feedbackconnection from the output of the third device to the cathode end of thecathode resistor of the second device, a connection from the controltube anode to the grid circuit of the first device and to the rectifyinganode of said first device, said connection causing change of the firstdevice's control grid potential when the control tube anode is at apotential less than a preassigned value and preventing said control gridfrom going positive when the control tube anode potential exceeds suchvalue.

' JAMES O. EDSON.

